The present invention relates to apparatus for regulating the delivery of power from a DC power source to a passive load such as a resistive load or to an active load such as a battery. It is considered particularly suitable for but not limited to applications where it is desired to operate a DC power source such as a solar array at or about its maximum power point.
A variety of systems have been designed to operate DC power sources at or near their maximum power point (xe2x80x9cMPPxe2x80x9d) with maximum power point tracking (xe2x80x9cMPPTxe2x80x9d). However, existing designs have been found to have various limitations. In some cases, designs are based upon an assumed set of operating conditions that may not always exist and they are not well adapted to adjust to differing operating conditions. For example, some solar array modules require the user to manually set what voltage the MPP should be at. Typically the voltage may be rated at 25xc2x0 C. However, for differing illumination levels or when the array is heated up, the required voltage can change quite significantly. In other cases, designs may not be well adapted to handle differing types of loads.
Many designs require the concurrent measurement and processing of both current and voltage parameters from which the calculation of power values are then performed to direct MPPT. The need for such calculations can be considered undesirable because they add to complexity and can slow system performance and accuracy.
A primary object of the present invention is to provide new and improved apparatus for regulating the delivery of power from a DC power source to a load while dynamically and accurately tracking the MPP of the source under a wide range of operating conditions.
A further object of the present invention is to provide new and improved apparatus of the foregoing type which requires the measurement of either output voltage to the load or output current to the load, but not both, to achieve MPPT.
In a broad aspect of the present invention, there is provided apparatus for regulating the delivery of power from a DC power source (for example, a solar array) to a load, such apparatus including a switch mode DCxe2x80x94DC power supply (xe2x80x9cSMPSxe2x80x9d), a voltage sensor operatively connected to the output of the power supply, a current sensor operatively connected to the output of the power supply, and a controller for providing a pulse width modulation (PWM) signal to the power supply. The PWM signal has a controllable pulse width or PWM on-time.
The power supply, which is controllable in response to the PWM signal, includes an input connectable with the source for receiving power from the source, an output connectable with the load for delivering power to the load and a PWM input for receiving the PWM signal.
The voltage sensor provides a first control signal corresponding to the voltage sensed at the power supply output. Similarly, the current sensor provides a second control signal corresponding the current sensed at the power supply output.
The controller is operatively connected to the PWM input of the power supply and to the sensors for receiving the first and second control signals and for providing the PWM signal to said power supply. It includes:
selection means for selecting whether to control the power supply by tracking output voltage or by tracking output current;
signal processing means for repetitively computing from the first control signal an average voltage value representative of average voltage at the output of the power supply;
means for comparing the average voltage value as computed on each repetition with the average voltage value as computed on each immediately preceding repetition;
signal processing means for repetitively computing from the second control signal an average current value representative of average current at the output of the power supply;
means for comparing the average current value as computed on each repetition with the average current value as computed on each immediately preceding repetition;
means responsive when the selection means has selected output voltage tracking for increasing the PWM on-time if the average voltage value as computed on a given repetition is greater than the average voltage value as computed on an immediately preceding repetition, and for decreasing said PWM on-time if the average voltage value as computed on a given repetition is less than the average voltage value as computed on an immediately preceding repetition; and,
means responsive when the selection means has selected output current tracking for increasing said PWM on-time if the average current value as computed on a given repetition is greater than the average current value as computed on an immediately preceding repetition, and for decreasing said PWM on-time if the average current value as computed on a given repetition is less than the average current value as computed on an immediately preceding repetition.
The foregoing apparatus provides MPPT operation which serves to maximize the output power from the power supply to the load. By maximizing the output power (which is the real object of MPPT operation), the input power from the DC power source is inherently maximized. Hence, the apparatus serves to operate the DC power source at or about its maximum power point.
It will be noted that advantageously only one of the output parameters (output voltage or output current) needs to be tracked. The decision as to which parameter to track is made by the selection means. If the load is a battery or a relatively low impedance load, then output current preferably is selected and tracked because output current will be dominant output parameter. Similarly, if the load is a relatively high impedance load, then output voltage preferably is selected and tracked because output voltage will be the dominant parameter.
Further, the monitoring of power supply output parameters as opposed to power supply input parameters avoids the need to actually calculate the power level. If control was based upon power supply input parameters, then MPPT operation would require both input current and input voltage to be tracked and multiplied thereby complicating and slowing the process, and leading to potential inaccuracies.
In a preferred embodiment, the controller includes means for computing from the first control signal a first normalized value representative of the power supply output voltage, means for computing from the second control signal a second normalized value representative of the power supply output current, and means for comparing the first and second normalized values. If the load is a battery, the selection means is operative to select output current tracking and the comparison of normalized values need not be made. However, if the load is not a battery, then the selection means is responsive to the comparison to select output voltage tracking if the first normalized value is greater than the second normalized value, and to select output current tracking if the first normalized value is less than the second normalized value. As discussed below in more detail, the base of normalization for output voltage is the maximum rated voltage that the power supply is designed to deliver. Similarly, the base of normalization for output current is the maximum rated current that the power supply is designed to deliver.
The foregoing and other features and advantages of the present invention will now be described with reference to the drawings.